Setting correct content type for docx files attached with mml-attach-file?

[HGV]

When I attach a docx file using mml-attach-file, emacs/mml does not correctly 
detect the content type from the extension. PDFs work just fine. I believe the 
problem is simply that the set of content types doesn't in fact have an entry 
for docx (which should be 
application/vnd.openxmlformats-officedocument.wordprocessingml.document). I 
cannot, however, figure out how this list of types is generated. It currently 
has 69 entries.

Where is the MIME content types list for mml-attach-file coming from and how do
I modify it? Or have I misidentified the problem and there is another obvious 
culprit?

I'm using GNU emacs 28.1 (with doom 3.0.0-dev) on MacOS 10.15.7. I use notmuch 
for my email, calling mml-attach-file from the notmuch message mode.